1987 mercedes benz560sel rough idle

Hot or cold car always starts,Idles perfect , then RPM drops,Idles rough and stalls.Econometer moves to Red area.car will easily restart and drives well,it also idles well.Once engine is shut down ,even for a short while,It does the same thing again.car is used for short trips.I live in the US Virgin Islands.AMbient Temperature 80 degrees.Once running car is perfect at all speeds and idlles smoothly.
Hero 1987 MB 560sel

I’m afraid you may need a MB specialist. It appears from what I was able to dig up that this vehicle has an unconventional fuel metering system. Remember that the engine operates at 200F+, so 80 degrees is cold by engine standards.

I know there are MB guys on this forum. Perhaps one can help. Or, perhaps a visit to a MB specific forum would yield good guidance.

Not familiar w/how this engine handle idle control, but cars of that era usually have a device that opens a passage between the air cleaner and the intake manifold as a function of coolant temperature, more open when it is cold, then less as the engine warms up. It may be that isn’t working, or the throttle body itself is dirty. Just a guess is all, lots of other things can cause this symptom. To fix it properly you’d need a Mercedes experienced mechanic to take a look.

My deepest condolences as your car has CIS fuel injection; a.k.a., The Devil’s Workshop. :slight_smile:

These CIS systems are very touchy. They will NOT tolerate any kind of air leak or crankcase leak. That means even the oil dipstick has to be fully seated in the tube.
It could also be a fuel related problem due to the fuel pump, fuel distributor, injector(s) leaking off, etc.

Generally speaking, the first step when a CIS problem crops up is a fuel pressure test. If there is no Benz specialty place in your area someone who handles older VWs, BMWs, etc may be able to do this test as those cars used CIS also and the specialized fuel pressure gauge along with principles of testing are the same. Best of luck.

What @ok4450 said. Given that, my first guess would be one of the 459 feet of vacuum hoses or 321 couplings has sprung a leak…

As a former Benz master tech, I’ll say a few things

The car does indeed have an idle air control valve

Vacuum leaks caused many of the problems on these CIS engines . . . then again, vacuum leaks cause problems on many types of engines. But the CIS engines seem particularly sensitive to vacuum leaks

The fuel system in general was somewhat problematic. Common problems were leaking injectors and accumulators

I would ask a shop to use an evap/smoke machine to check for vacuum leaks. You might be surprised what you find

A fuel pressure test would also be a good idea, as the other guys said. But you better find somebody that knows how to test it. These systems don’t have a fuel rail with a test port

Make sure the engine coolant temperature sensor is operating correctly

Oh, CIS is involved then? I have some experience w/CIS. One thing I learned early on was that the CIS fuel injection system on my late 70’s VW Rabbit was very sensitive to fuel quality. When it started to misbehave and it had fuel related symptoms the first thing I’d do is take the fuel accumulator apart and clean all the internals. If your CIS is like my Rabbit’s was, there’s a metal plunger that meters out the gas to the injectors that must be spotlessly clean; likewise the bore it slides in. One good thing about it, on the Rabbit at least this was a fairly easy and quick task, once you know how it’s done.

BTW OP, if this symptom started soon after replacing the fuel filter, that would confirm you may have sand, dirt, or sludge in the CIS fuel accumulator.


The accumulator can’t be taken apart on this vehicle

I suspect you may be talking about the fuel distributor

You are right db4690, thanks for pointing that out. It was the" fuel distributor" that required cleaning. I’m not sure the Rabbit even had a fuel accumulator now I think of it. What is the purpose of the Benz CIS fuel accumulator? All that was so long ago I have forgotten the nomenclature on the parts I guess.

Yes, the Rabbit has a fuel accumulator and it has 2 jobs. One is to maintain fuel pressure when the engine is off and the other is to smooth out fuel pressure pulses. The latter is critical because of the much higher pressures that CIS operates at and due to the fact that the injectors spray constantly whenever the engine is running.
Most injection runs at 35-40 PSI or whatever and CIS runs at 70ish; although it’s usually figured in Bars.

With the fuel distributors, the usual problem with those is when the pressure relief valve acts up. That’s often cured by removal and cleaning.
Anytime one of those fuel distributors is taken apart extreme care must be used to not damage any parts and to avoid any contamination at all.
The fuel distribution slots and so on are machined with a laser and fit is down to the ten thousandth of an inch. One nick or speck of dirt…

The fuel accumulator’s purpose, if I recall correctly, was to enable easy starting

Unfortunately, it often was the cause of many problems

Thanks to all who gave such clear advise.I should have said there is a Hiss from the area of the fuel Pumps.Car has been running fine with this noise for several years.Why does car run and idle fine after the initial start-up problem? It restarts at once and usually runs OK.Thanks for your patience Hero 1987 MB 560sel

Don’t worry about the hiss

The fuel pumps are external . . . meaning they’re not in the tank, unlike newer cars

That is why you can hear them

For what it’s worth, if the car has the original pump it would not surprise me if the pump was on shaky ground as those pumps spin at very high RPMs under a much higher pressure load. In the past I’ve cut a few questionable Bosch pumps open (running cars) and found the commutators on the armatures to be horribly worn along with the end bushing near the pump impeller.

Regarding those accumulators, I’ve seen several hard starting VWs in which everything checks out and the problem was solved with a new accumulator.
The irritating part was that the VW factory rep would insist until hxxx froze over that the accumulator had zero/zilch/nada to do with starting.
Well, the accumulator fixed the problem so…

I was doing some reading the other day . . .

It seems that Bosch introduced the L-Jetronic and K-Jetronic at the same time, give or take a year or 2

I personally know some of the quirks of the K-Jetronic system

The L-Jetronic seems much more like a conventional EFI system, except that it used a volume air flow sensor, with the movable vane, instead of the later MAF

Do any of you guys have stories about L-Jetronic?

VW used the L for quite a while. This is ancient history but problems were generally limited to air leaks, temperature sensors, and problems with the Air Flow meters.

The AF meters would sometimes develop problems in the printed circuit and in the event of a backfire the flap in the meter could be blown out.

Hero 1987 MB Rough Idle.Here is another clue.After car starts and run fine for about one minute or more there is a noise(click)from the top,right front of engine.Then theRPM drops and engine stumbles and stalls.restarts instantly and runs fie or may stumble againshortly after.THANKS FOR youy patience

Db4690 is the Benz guy so I will defer to him. My experience has been with SAAB and VW. The systems are similar but maybe the Benz has a few tweaks that I’m not aware of.

Theorizing only, I might suggest that the click could be an Idle Air Control valve that had a hiccup which then caused the stumble. Another theory could be an errant spark jumping from a plug wire, etc due to a misfiring spark plug as that could also cause a stumble.

The car does have a regular idle air control valve

They did occasionally gum up

The ignition coil would sometimes fail when hot. They would pass all kinds of tests cold, but would leave you stranded

As far as the plug wires go, the car is probably in need of a set, because of its age

Besides the CIS system, this is just an old school car with a distributor, plug wires, idle air control valve

If it was my car, I’d take George’s approach, and make sure all the tune up stuff was up to date

Then I’d make sure the ECT sensor was working correctly. Then I’d smoke test the intake system to look for unmetered air

I certainly hope nobody’s messed around with the throttle linkages. I learned that if you don’t have the factory service information, you’ll most likely adjust everything incorrectly

I think the key to fixing this car is not so much expertise with the fuel injection system, but sticking to the basics and making sure all the tune up stuff is up to date and in good shape

I’ll add my opinion about parts . . . these older Benzes often don’t like cheap aftermarket parts, even if the aftermarket parts happen to be cheap German aftermarket parts.

I remember an instance where an otherwise pristine 380SL wouldn’t start without bridging the fuel pump relay socket contacts with a banana plug lead, and then it wouldn’t idle worth squat. It turned out the brand new aftermarket German fuel pump relay and idle air control valve were both defective. After installing genuine Mercedes-Benz parts, both problems were resolved. I advised the customer to return to the independent shop which had recently installed these aftermarket parts and explain the situation.

I know I was rambling, but perhaps there was something useful in there somewhere . . .


For the record, I always hated working on driveability complaints on those CIS cars

Not surprisingly, a lot of the other guys also hated it, including the older guys who were around when these cars were introduced

I suspect the attitude was similar at VW and Saab dealerships